Reduction of waste biosolids by RAS-ozonation: Model validation and sensitivity analysis for biosolids reduction and nitrification Siavash Isazadeh, Luis Enrique Urbina Rivas, Pinar Ozdural Ozcer, Dominic Frigon * Department of Civil Engineering and Applied Mechanics, McGill University, 817 Sherbrooke Street West, Montreal, QC, H3A 0C3, Canada article info Article history: Received 3 April 2014 Received in revised form 22 October 2014 Accepted 11 November 2014 Available online 20 December 2014 Keywords: Activated sludge ASM3 model Biosolids reduction Global sensitivity Nitrification Ozonation Sludge abstract Biosolids reduction model by return activated sludge ozonation was validated by simulating nitrification data compiled from our pilot-scale and the literature studies. Then, a global sensitivity analysis (GSA) was performed to identify influential and non-influential parameters for biosolids reduction efficiency, change in specific nitrification activity (SNA), and alteration to expected nitrification stability. In general, the model outputs were sensitive to operational and ozone reaction parameters, but not to biochemical parameters. For operational parameters, mainly temperature and initial solids retention time (SRT) influenced all model outputs. For biosolids reduction, increase in the degradability of the influent COD decreased the reduction efficiency. For SNA, the changes were highly dependent on the influent TKN/ COD ratio. Our findings also imply that the stability of the nitrification process in ozonated systems should be enhanced at constant MLVSS for warm temperatures, but could be reduced at temperatures below 12  C and aerated SRTs below 10 days. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Management of waste biosolids is one of the main operational costs at wastewater treatment plants (WWTPs), and drives facilities to adopt new technologies to reduce biosolids production. Partial ozonation of return activated sludge (RAS-ozonation) is one of these technologies. During RAS-ozonation, chemical oxygen de- mand (COD) is solubilized, microbes are inactivated, and non- biodegradable particulate organics are transformed to biodegrad- able substrates (soluble and particulate) and non-degradable sol- uble COD (Foladori et al., 2010a). The resulting COD pools are either re-consumed by the biomass or leave the system with the effluent. Therefore, biosolids reduction is achieved by the synergy between the ozone reactions and the biological processes (Paul et al., 2012). The wide range of biosolids reduction performance reported in the literature (from 3.7 to 10.4 g-TSS reduced /g-O 3 , dosed (Foladori et al., 2010a)) makes it difficult to predict the outcome of new in- stallations. Such disparity in performance stems from variable RAS- ozone contactor characteristics (e.g., ozone transfer efficiency and contactor configurations) (Chu et al., 2008), and variable waste- water treatment operational conditions (e.g., wastewater composition, treatment process, and solids retention time [SRT]). Therefore, reported performance values mix two distinct aspects of process modeling: RAS-ozone contactor performance and biolog- ical treatment characteristics (Fig. 1). Alternatively, in this study we model these two types of components separately. We recently introduced an extension of the International Water Association Activated Sludge Model 3 (IWA-ASM3) to describe ozone reactions with suspended solids (Isazadeh et al., 2014). In this extension, instead of modeling the extent of ozone reactions in the RAS-ozone contactor which widely range between 0.7 and 9.6 g-COD/g-O 3,dosed (Labelle et al., 2011), the model directly uses the amount of COD solubilized in lieu of the ozone dose. A major goal of the current study is to conduct a global sensitivity analysis (GSA) of the biosolids reduction performance to better understand how the local wastewater treatment conditions affect biosolids reduction performances. In addition, we provide a better understanding of the impact of RAS-ozonation on the nitrification process. The vulnerability of nitrifiers to partial RAS-ozonation has been evaluated in long-term monitoring of (i) nitrification process efficiency and (ii) specific nitrification activity (SNA) studies. However, a number of in- consistencies arose from these studies. First, while the nitrification process efficiency (measured as the residual ammonia in the bioreactor effluent) was typically not affected by RAS ozonation * Corresponding author. Tel.: þ1 514 398 2475. E-mail address: dominic.frigon@mcgill.ca (D. Frigon). Contents lists available at ScienceDirect Environmental Modelling & Software journal homepage: www.elsevier.com/locate/envsoft http://dx.doi.org/10.1016/j.envsoft.2014.11.009 1364-8152/© 2014 Elsevier Ltd. All rights reserved. Environmental Modelling & Software 65 (2015) 41e49